USB voltage regulators operate with extremely low equivalent series resistance (ESR) values, for example around 10 milliohms. This ESR value is not very accurate because the ceramic capacitor manufacturers rarely specify the value of ESR in their data sheets. Additionally, the exact value depends on the size/voltage rating design of the external capacitor, and can vary based on user connection topology (circuit board layout and components used thereon), e.g., putting multiple small capacitors in parallel, thereby reducing ESR; or using thin/long printed circuit board traces to connect to the capacitor, thereby adding copper trace resistance ESR. Typically the externally compensated low-drop out (LDO) voltage regulator uses large value external capacitors for stability with a limited range of ESR ratings. The ESR values will heavily impact the design of LDO voltage regulators due to its variable zero pole locations depending upon the load conditions. Therefore existing products having a USB interface use standard voltage regulator architectures that require very large external capacitor sizes with variations of ESR values in order to keep the voltage regulator stable.
Referring to FIGS. 1 and 1A, depicted are various frequency response graphs showing different ESR situations. FIG. 1(a) shows a desired normal frequency response. FIG. 1(b) shows a situation where the ESR is too high. FIG. 1A(c) shows a situation where the ESR is too low. And FIG. 1A(d) shows ESRs resulting in the dreaded “tunnel of death.”